Electrical communication system



`Fuly 9, 1940. H. o. ELLIS Er AL 2,207,537

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ELECTRICAL COMMUNICATION SYSTEM A Filed Nov. 27, 1937 e sheets-sheet s [Jal )m7 W2 W HH H zsl /A O L 5 D mum/mkg Patented July 9, 1940 ELECTRICAL COMMUNICATION SYSTEM Harold Owen Ellis, Stoneleigh, and William Henry Bernard Cooper, Ruislip, England Application November 27, 1937, Serial No. 176,804 In Great Britain December 3, 1936 1o claims. (ol. 17a-L16) The invention relates to improvements in or relating to electrical communication systems, for

instance, telephone or like exchange systems.

In the transmission of direct current digital impulses over a long line the characteristics of the line tend to mutilate the wave shape of the impulse to such an extent that incorrect operation of apparatus at the receiving end of a line may occur. It has also been lfound that the inn sertion of transmission bridges at the outgoing and/or incoming ends of long lines contribute to the mutilation of the wave shape of the impulses transmitted over the line.

The object of the invention is to provide a signalling system in which the impulses received by a receiver at the incoming end of a long line are substantially ci the same wave shape as the impulses transmitted from the outgoing end of the line.

The main feature of the invention comprises an electrical communication system in which di rect current digital impulses are transmitted over a metallic loop circuit, characterised in this that the impulse transmitting equipment comprises means for generating impulses the growth and decay curves of which are substantially symmetrical while the impulse receiving equipment comprises a transformer designed to produce steepfronted voltage pulses which are substantially of the same magnitude and shape.

Another feature of the invention comprises an electrical communication system in which direct current digital impulses are transmitted over a metallic loop circuit, characterised inthis, that the impulse transmitting equipment comprises battery and earth connections for the two sides of the loop and normally-closed impulse springs which are connected in bridge on the loop circuit and which are opened to allow a direct current impulse to be transmitted from said battery and earth connections and that the impulse-receiving circuit is connected in bridge on the loop circuit whereby the growth and decay curves of each impulse transmitted are symmetrical.

A metallic loop comprises two lines each of which may comprise one or more conductors con- V nected in parallel.

Figs. 3 and 3A show the equipment at the outgoing end of a long line in the preferred embodiment of the invention. l

Figs. 4 and 4A show the equipment or the incoming end of the line.

Referring now to Fig. 1, LI and L2 are `the leads from a calling subscribers or calling operatorsv line. Relay A connected to both legs of the line LI, L2 responds to incoming impulses and contacts thereofr (not shown) connect up an impulse storage device, and other contacts al of relay A operate a slow-to-release relay B which remains operated during impulsing. When the impulse storage device is 4taken into use a relay Y (not shown) is operated and remains operated until all impulses stored in the storage device have been sent out. Over contacts y2, the circuit of relay M is completed, relay M operates and via con` tacts m2 completes the operating circuit of relay N which operates and locks via n. Meanwhile the operation of relay B completed a circuit via bl for relay AA at the incoming end of the line. Thus the transmission bridge comprising IL and D is connected to the outgoing lines L3 and L4, which may be lines of an interexchange junction.

Upon the operation of relay N, however, the contacts n2 and n3 disconnect the speech transmission bridge from the lines L3 and L4 and con'-v nect the outgoing line L3 via resistances e and f to earth and the outgoing line L4 via resistances h and gl to battery. The battery feed for the speech transmission bridge at the outgoing end of the line is thus prevented from mutilating transmitted impulses, and the loop circuit over the outgoing lines L3 and L4 is maintained via the resistance network.

The impulse storage device controls the transmission of impulses over the outgoing lines by the normally closed contacts ip. When relay M is operated, contacts mi are open and a negative battery is applied to line L3 via gl, impulse contacts ip, resistance e, contacts n2 and icl, and to line L4, via resistance gl, h, contacts n3, 165.

The negative potential on line L4 operates relay X via normal contacts myZ, rl. Contacts xl close and relay XY operates, its contacts yl and :11112 changing over and disconnecting the incoming lines L3 and L4 from the receiving battery feed speech transmission bridge and connecting them to the impulse receiving circuit. Relay X releases but relay XY remains locked Via contacts r3 and $113.

Relay YA operates via contacts :m4 and bc4 and over its contacts ya2 .energises the f winding sti of the telegraph relay P. The contact :r1/5 rconnects up the anode circuit of the valve V and Contact y closes the circuit of a high speed relay HA which is thereby placed under the control of the contact pl. Contact :ryl also provides a holding circuit for relay BB which operates via contact :cg/3 closed to earth. Contact yall prepares an impulsing circuit over lines L5 and L5 to a subsequent stage in the connection to be set up.

The flux produced by the winding f of the relay P is arranged to neutralize the flux produced:

by the normal anode current flowing through the winding e of the relay when the circuit is ready to receive impulses.

The relay operations described above occur before the relay M has time to release due to the opening of its energising circuit at n4.

When the impulse storage device transmits an impulse, contacts ip, which are normally bridged across the line, are opened and disconnectthe negative feed from line L3 which is now connected via n2, e, f, mi tolearth.

A circuit is now completed via line L3, contacts yl, Fig. 2, inductance L, primary winding Tl of a transformer, inductance contacts licfl, .rg/2, line L4 contacts yl, Fig. l, lol, resistances h, and g to negative. An impulse of direct current is thus transmitted over the line and received by transformer primary winding Tl each time contacts ip are opened and .,closed, the transformer primary winding being so designed that each impulse has a steep Wave iront and is symmetrical in wave form. The secondary winding of the transformer is connected to a high impedance impulse responsive device. In the embodiment shown in Fig. 2, the impulse responsive device is a valve and the secondary` winding of the transformer is arranged to decrease the bias voltage applied to the grid of the valve V as the impulse current in thelines L3 and L4 rises at the beginning of an impulse and to increase the bias voltage applied to the grid of the valve as the impulse current in the lines L3 and L4 falls at the end of an impulse. The inductances L and MM are inserted in the line to lter out high voice frequencies which may accidentally be picked up by the loop circuit.

The voltages generated by the winding T2 are applied to the grid of valve V and at the beginning of an impulse due to the bias voltage on the grid being decreased, increased current ows in the anode cathode circuit and through winding e of the polarized relay P. The windings g and h are holding windings of the relay P and are arranged to hold the armature of the'relay in the position to which it is driven by the action of coils e and f, but the unbalance of ilux due to windings e and f which produces the armature movement is greatly in excess of that due to either of the windings g or Zi. The increase of current in the Winding e causes the relay P to operate and close an operating circuit for relay HB via pl and ya3. Quick acting relay HB operates via contacts pl and contacts f'zbl change-over so that the flux through the l?. winding maintains the armature of relay P in its operated position. Contact pl opens the operating circuit of a second quick acting relay HA which breaks the impulse circuit over lines L5 and L6.

At the end of an impulse the bias voltage applied to the grid of valve V is increased, lthe anode current decreases and the iiux due to the coil f then being greater than the flux due to the decreased anode current owing through the winding e, the relay P releases. Contacts pl change-over and release HB, the contacts hbl connecting up the winding g of the relay P, the armature of which is thereby maintained in its normal position until the next impulse is received. The return of contact pl to its normal position reoperates the relay HA which via contacts hal closes the subsequent circuit and completes the transmission contact of the impulse. The provision of resistances S and W, and condensers HH and GG is made in order to further contrcl the time intervals of the impulses transmitted to the subsequent circuit. The condenser GG and resistance S are arranged to control the operating and release times of relay HA whilst resistance W and condenser HH control the rise and fall of current in the subsequent impulse circuit.

The operations which take place in reconnect ing the speaking bridge will now be described.

When contacts pl rst operated at the beginning of the impulse, relay C operated from battery via yai and pl to earth. Relay C is a slowto-release relay and remains operated during the reception of all the impulses in an impulse train.

Contacts clclose and operate relay BC which is also a slow-to-release relay and which also remains operated during the reception of all the impulses in an impulse train. Contacts c3 maintain the impulse receiving circuit which was l.

originally connected via contacts no3 in their normal position, whilst contacts cli maintain the circuit for winding f of the relay P, which was originally energised over contacts Dcd.

At the endof the reception of a train of digital impulses relay C releases -and via contacts c2, oe3 and xy. a positive battery is extended to the incoming line L4. If all the impulses originally stored at the outgoing end of the line have been transmitted, contacts yl open and release the short circuit on relay K which operates to the positive potential on La and locks via h5. 'I'he operation of relay K releases relay N at contacts k3, and via normal contacts nl, and operated contacts kl a positive potential is applied to line L3. The release of relay C removes earth from the relay YA which releases and at yal closes an operating circuit for relay R, positive line L3, :cg/l, inductance L, transformer primary Winding Tl, inductance MM, c3, rectifier KR, contacts hcl, ya, R, earth. Relay R locks via r2 and bc2 to earth, and is released upon the completion of the release of relay BC.

The operation of relay R opens the locking circuit of relay XY at 'r3 and the contacts :ryl and myZ change-over and reconnect the speech transmission bridge to the incoming line. The positive feed to line Lil is maintained via contacts rl and .721,12 until relay R is released when contacts be2 return to normal. With the removal of the positive potential from lead L4 relay K, Fig. 1, releases and reconnects the speech transmission bridge at the outgoing end of the line L3, L4. The above arrangement of relays ensures that the speech transmission bridges are not reinserted in the lines until after the expiration of a certain time, after the end of transmission of an impulse train or a series of impulse trains.

If, however, there are still a number of impulses stored in the impulse storage device contacts yl remain closed and relay K does not operate so that the release of the XY"relays to reconnect the speech transmission bridge to the lines L3, L4, does not take place.

which allows the incounting portion of the -me Referring now to the preferred embodiment of the invention as shown in Figs. 3, 3A, 4 and 4A. Figs. 3 and 3A show the outgoing end of an intereXchange junction whilst Figs. 4 and 4A show the incoming end of the same interexchange junction line.

I'he circuit is shown connected to an operators cord circuit but it is within the scope of the invention that the line may be connected to an automatic switching train.

When the operator plugs in to the jack J, relay S receives a negative feed from the ring of the operators cord circuit. Relay S operates and at sl completes the operating circuit for relay SS which operates and lock via ss5 to earth on normal contact ill. Contact ssl connects flicker earth from FEL to the operators cord circuit, this flicker earth being used at the operators position to indicate by means of a lamp that the circuit is ready to receive dial impulses. Contact ss2 extends negative battery to relay L at the operators position which operates and lights a busy lamp.

'I'he operator thereupon throws the dial key in her key set and extends negative potential Via the tip conductor to operate the relay RR. Relay DT operates via rrl, Zr4, S83 to ground, locks Via dtd and at dt disconnects the flicker earth from the operators sleeve Wire and connects battery to relay LR. Relay S- remains operated, but LR does not operate since battery is connected to it via a high resistance.

Upon the operation of DT, contacts dt! and t2 change-over and release relay RR. Contacts dt3 extend the loop forward to wire L3 of the outgoing junction line. Relay A operates over the contacts dtl and dtZ, after the operator has removed negative from the tip and substituted a normal feed followed by sloW-to-release relay B which operates Via al.

Relay IL operates to battery feed on the line L3, back Contact dd?, Fig. 4, relay AA, contacts :r1/l, line L3, nl, Fig. 3A, dtS, rectifier RC2, IL. n2, line L4, myZ, Fig. 4, yal, relay X, ddl, bri, earth.

Relays AA and X, Fig. 4, operate followed by relay BA, Fig. 4A, earth, Fig. 4A, xi, dal, rye, Fig. 4, bb2,-Fig. 4A, winding BA, battery. Relay BA closes contacts bal and extends loop forward to an jautomatic subscribers circuit over lines L6 and L5. Relay I operates from L5 Via RC4, Fig. 4, I,`3Fig. 4A, yaZ, bai, yal to LS.

The operator then proceeds to dial the digits of the required subscribers number. The iirst impulse received over Wires L! and L2 releases A which returns its contacts al to normal. The slow-to-release relay C, Fig. 3A, operates via b2, Fig. 3, dt, Zr, al, both this relay C and the slowto-release relay B, Fig. 3, remaining operated during the reception of an impulse train.

Contacts c3 close and operate slow-to-release relay IP, Fig. 3A, which energizes the slow-torelease relay IS, Fig. 3A.

When C is energized the rotary magnet RM, Fig. 3, is energised and steps the incoming portion of a mechanical impulse register according to the number of impulses received in each impulse train. As soon as RM steps theregister off-normal, contacts NS close and remain closed until the outcounting portion of the register reaches the position to which the incounting portion is stepped in response to impulses. When contacts c2, Fig. 3, closed, the magnet MM energised,opened the springs Mdm, Fig. 3A, and

lifted the pin setting levers PSL (not shown) ehanical register to rotate under control of RM without depressing the pins spaced around the periphery of a Xed disc. At the end of each impulse series relay C releases and the PSL levers return to normal position and operate on a pin in the xed disc to mark the digit received.

At the end of each impulse train the Mdm contacts are reclosed and thus at the end of the first such train relay BY operates; battery Mdm, NS, earth. Relays N and DE operate in parallel Via byl and Slim, relay N locking via 6315 and n4. Relay IP and then relay IS also operate as a result of the operation ofA contact 22116. The operation of contacts nl and n2 disconnect the speech transmission bridge from the outgoing junction line and connect an impulse generating circuit comprising springs YIlVlPS, which are normally closed and bridged across the loop resistances Rl, R02, RDS, inductances Ll, L02 and condensers CCI and CCE to the outgoing lines L3 and L4, the impulse generating circuit being designed to give a symmetrical wave form to the impulses.

Relay Z operates over the loop formed bythe closure of contacts nl and u2 and at al closes the operating circuit for relay CX which locks via ded, and 0:64. Contacts casi and cZ changeover and release relay Z and the outgoing loop circuit is broken at hr3. The operation of con tact Cat5 releases the slow-to-release relays IP and IS in turn. A circuit is therefore closed from earth via springs NS, Fig. 3, z'sI, Fig. 3A, tpl, Fig. 3, to magnet SM which is energised and steps off the outcounting impulse wheel and'opens its oif normal contacts Sdm, Fig. 3A. The opening of contacts Sdm releases relay DE but relay N remains operated over its locking circuit.

The operation of CX opened the loop circuit extending over the outgoing line, and relays AA and X Fig. 4 release. When fri, Fig. 4A, reaches its normal position earth is extended via rl, bad, bc3, dati, bril to operate relay XY. Contacts .ryl and :r1/2, Fig. 4, change over and disconnect the incoming lines L3 and L4 from the battery feed circuit and connect them to an impulse receiving circuit IRC comprising inductances, condensers and resistances and the primary Winding of transformer T. The secondary winding of the transformer is connected to a valve V. The valve has a variable resistance R5 connected in the cathode grid circuit. This feed back resistance is used to regulate the normal current flowing in the winding c of the contact making relay P, Fig. 4A.

The primary winding of the transformer T, Fig. 4, is so designed that each impulse transmitted over the lines AL3 and L4 has a steep Wave front. At the impulse transmitting end the inductances Lili and L02, the resistances Rill and R92, and the condenser CO2 comprise a filter circuit which cuts oi the higher voice frequency components of the impulse Wave generated.

The resistance R03, together With condenser CC! forms an impedance which has the same val-'- ue at voice frequencies as the impedance of the line, in order that the impedances should remain balanced when the change-over from the speech transmission bridge to the impulse generating circuit takes place, and so prevents the setting up howling.

Similarly at the impulse receiving circuit the condenser RCI connected in series with the resistance RRI gives an impedance equal to the impedance of the line circuit at voice frequencies and prevents the setting up of howling at the receiving end when the impulse receiving circuit is connected to the line in place of the normal receiving speech bridge.

The inductances LLI and resistances RRZ and RRS form a filter which cuts off the high voice frequencies at about 120 cycles per second, and so guards the impulse receiving circuit against stray voice frequencies.

Upon the operation of XY, Fig. l-A, earth is extended via bci and :c1/3 to relay YA which operates and prepares the circuit of the winding f of the polarized relay P, and contacts :r1/5 connect up the filament supply of the valve V. In this manner the winding f is not connected up until current surges received from the junction line are dissipated in the valve circuit.

Meanwhile at the outgoing end of the line the i magnet SM, Fig. 3, steps around and sends out impulses to the line L3, L4 by means opening the impulse springs IMPS, Fig. 3A. The springs Sdm, Fig. 3A, are opened as soon as SM steps oif normal, to open the locking circuit of relay DE which, being a slow to operate relay, remains unA operated during each impulse train. Contact dell therefore opens and releases relay CX which completes the impulse loop circuit at cmi and cm2.

The operation of relay P, Fig. 4A, is similar to that of relay P in Fig. 2. Relay P has four windings e, f, g and h, e being connected in the anode circuit of the valve V, winding f being designed so that when it is energised the flux due to it neutralises the flux due to normal anode current owing through winding c when the device is ready to receive impulses. The windings g and h are arranged to hold the armature of relay P in the positions to which it is driven by either the e or ,f windings.

Contacts pl control the operation of a very quick acting relay HS which controls the transmission of impulses over the line L5, LEB and operates the relay MD via ba4. The contact pl also controls the operation of a quick-acting relay HSB, contacts heb! of which change-over the holding windings y and h of relay P.

Associated with the impulse repeating contacts hsl are resistances H and W and condenser HH which are provided to exert a control on the duration of the impulses transmitted over lines L5 and LS.

During the interdigital pauses relay XY is released and relays X and AA test over the lines L3 and L4 to determine whether more impulses are to be transmitted. If the impulse transmitting condition still exists then. relays AA and X remain operated and relay XY reoperates to connect the junction line to the impulse receiving circuit.

At the end of impulsing i. e. when the impulse transmitter SM at the register has sent out as many impulses as the total impulses stored on RM, the two movable parts SM and RM of the register are in corresponding positions, the contacts NS open and release relay BY. Contacts lig/5 open the locking circuit of relay N which also releases.

At the incoming end of the lines L3 and L4 the slow-tod'elease relay C releases at the end of impulsing and is followed by relay YA. Contacts yaZ return to normal and reconnect relays D and I across the lines L5 and L6. Relay XY releases and at xyl, Xy2 disconnects the impulse receiving circuit from the incoming lines L3, L4 and reconnects the speech transmission bridge.

Relays AA and X Fig. 4 are once more connected to lines L3 and L4 and both operate.

If any further impulses are transmitted over lines L3 and L4 relays AA and X release as previously described, followed by the operation of relay XY. In this way it is ensured that whenever impulses are received over L3, L4 the speech transmission bridge at the incoming is disconnected from the incoming line.

When the operator has finished sending out impulses she restores her dial key to its normal position and battery is connected via a low resistance to the sleeve circuit. Relay LR op- 'lio crates to this low resistance battery and locks via T5 and resistance LL to battery. Relays DT and A release' in turn, and relay RR is recon nected to the line LI Earth is extended via contacts z'lz', dri, lf3, dt3, ssl low resistance winding of S to the sleeve circuit and at the operators position a lamp glows continuously to indicate the end of the transmission of impulses over the lines L3 and L4 to warn the operator to wait for a signal indicating the reply of the called subscriber. Contacts Z12 maintain the circuit of re lay IL when contacts dt open upon the release of DT.

When the called subscriber answers, reversal of battery occurs on lines L5 and L6 and current flows through rectier RC3 to operate relay D in series with relay I which is already operated by normal battery feed on the lines L5 and L6. Contact di completes the operating circuit of relay DA from earth via haii, di, md3; the relay MD being released by the operation of contact dl. Contacts da complete the circuit of relay DD which operates and locks via dd3, dl and ba to earth. Contacts ddl and ddZ change over the battery feed to lines L3 and L4 and relays AA and X remain operated by the reversed battery feed.

At the outgoing end of the lines L3, L4 relay D, Fig. 3, operates via rectifier RC2, relay IL remaining operated in series therewith, and over contact all the operating circuit of relay DR is completed. The contacts dri open the circuit to the supervisory lamp at the operators position and the supervisory lamp is extinguished to indicate that the connection has been set up to the called subscriber.

Upon the completion of Ithe connection the subscribers hang up their receivers and release of the speech transmission circuit takes place in wellknown manner. The provision of inductances in the primary winding circuit of transformer 'I in addition to smoothing the wave form of the impulses received renders the valves circuit more immune from extraneous line current variation and thereby permits of greater sensitivity in the impulse repeater without the danger of spurious impulses being transmitted on to lines L5 and L6.

The embodiments of the invention have been described in relation to a telephone exchange system but it is obvious that the method of generating impulses at the transmitting end and correcting their wave shape at the received end, can be equally well applied to telegraph systems.

The impulse receiving device has been shown to include a valve but it is within the scope of the invention that the impulse receiver should include a gas discharge tube or any high impedance impulse responsive device.

What is claimed is:

l. A signaling arrangement for automatic telephone systems in which direct current digital impulses are transmitted over a metallic loop cirrent to pass over said loop circuit from said concuit, comprising means for generating impulses the growth and decay curves of which are substantially symmetrical, said means comprising a battery feed and springs for normally preventing current flow therefrom over the circuit, means for opening said springs for transmitting impulses over said loop circuit from said battery feed, and impulse receiving equipment comprising a transformer designed to produce in response to said impulses steep-fronted voltage pulses which are substantially of the same magnitude and shape.

2. A signaling arrangement for automatic telephone systems in which direct current digital impulses are transmitted over a metallic loop circuit, comprising battery and earth connections for the two sides of said loop, normally-closed impulse springs which are connected in bridge of the loop circuit and which are opened to cause a direct current impulse to be transmitted from said battery and earthconnections, and an impulse-receiving circuit connected in bridge of the loop circuit whereby the growth and decay curves of each impulse transmitted are symmetrical.

3. A signaling arrangement for automatic telephone systems as claimed in claim l, wherein said 4means for generating impulses comprises battery and earth connections for the two sides of the loop and normally-closed impulse springs which are connected in bridge of the loop circuit and which are opened to allow a direct current impulse to be transmitted from said battery and earth connections and wherein said impulsereceiving circuit is connected in bridge of the loop circuit whereby the growth and decay curves of each impulse transmitted are symmetrical.

4. A signaling arrangement for automatic telephone systems as claimed in claim 2, further comprising filter equipment interposed between the battery and earth connections of the impulse transmitting equipment and the loop circuit for cutting off the higher voice frequencies of the impulse wave.

5. A signaling arrangement for automatic telephone systems as claimed in claim 2, wherein said transformer includes a secondary, further comprising an impulse responsive device of very high impedance connected to said secondary.

6. A signaling arrangement for automatic telephone systems as claimed in claim l wherein the means for generating inmpules and the impulse receiving equipment are proportioned to present to the impulsing loop circuit terminations each of which has the same impedance at speech frequencies as the impulsing loop circuit itself.

7. A signal arranged for automatic telephone systems comprising a metallic loop circuit eX- tending from a sending to a receiving station, a battery connection to one side of the loop at the sending station and an earth connection to the other side of the loop at the sending station, impulse sending springs normally shunting said loop circuit, means for intermittently operating said springs to cause symmetrical impulses of curnections, and a receiving device in loop circuit responsive to said impulses.

8. In a telephone system, a link circuit extending from a line, talking feed bridges in the link circuit, an impulse sending circuit comprising battery and earth connections and an articial line normally disconnected from the line, a receiving circuit also comprising an articial line and a receiving device, means responsive to the transmission of a signal over the link circuit to substitute said sending and receiving circuits for the talking feedbridges thereof, means in the sending circuit thereafter responsive to an open- .ing of the line to cause said sending circuit to transmit an impulse of current over the link circuit from said connections, and said receiving circuit responsive to said impulse of current.

9. A signalling system for automatic telephone systems in which direct current digital impulses are transmitted over a metallic loop circuit, comprising battery and earth connections for the two -sides of said loop, normally closed impulse springs connected in a bridge of the loop circuit and which are opened to allow a direct current impulse to be transmitted from said connections, an impulse receiving circuit connected in bridge of the loop circuit whereby the growth and decay curves of each impulse transmitted are symmetrical,

thereto having an anode-cathode circuit, a contact making relay having a plurality of windings one of which is included in the anode-cathode circuit, and a second one oi which is connected to cancel the magnetic flux produced in the relay by current flowing in the anode-cathode circuit when the circuit is ready to receive impulses.

10. A signalling system for automatic telephone systems in which direct current digital impulses are transmitted over a metallic loop circuit, comprising battery and earth connections for the two sides of said loop, normally closed impulse springs connected in a bridge of the loop circuit and which are opened to allow a direct current impulse to be transmitted from said connections, l

an impulse receiving circuit connected in bridge of the loop circuit whereby the growth and decay curves of each impulse transmitted are symmetrical, said receiving circuit comprising a transformer, and a thermionic valve connected thereto having an anode-cathode circuit, a contact making relay having a plurality of windings one of which is included in the anode-cathode circuit,'a second one of which is connected to cancel the magnetic ux produced in the relay by current owing in the anode-cathode circuit when the circuit is ready to receive impulses, and two additional ones of which are connected for holding the relay armature in opposite contact making positions.

HAROLD OWEN ELLIS.

WILLIAM HENRY BERNARD COOPER.

said receiving circuit comprising a transformer, and a thermionic valve connectedv 

